1. Field of the Invention
The present invention relates to a process for preparing a superconducting thin film of oxide, more particularly, to an improved process for depositing a thin film of superconducting compound oxide on a substrate by Molecular Beam Epitaxy (hereinafter, MBE) technique.
The present invention relates also to a system for carrying out the process automatically by a computer.
2. Description of the Related Art
Thin films of compounds are prepared by so-called reaction-evaporation method in which materials are evaporated in an active gas atmosphere so that vapors of the materials are reacted with the active gas on a substrate to deposit a thin film of the compound on the substrate. When the compound is an oxide, oxygen is usually used as the active gas. In Tatsumi et al. "Epitaxial film growth of artificial (Bi--O)/(Sr--Ca--Cu--O) layered structure", Appl. Phys. Lett. 54 (23), Jun. 5, 1989, p. 2364-2366, an oxide superconducting thin film of Bi--Sr--Ca--Cu--O system is grown epitaxially by ion-beam sputtering technique in the presence of oxygen gas. It is also proposed to use excited oxygen which is activated by microwave in order to obtain an oxide thin film of high quality.
Thin films of compounds composed of more than two elements am advantageously prepared by the MBE technique in which proportions of constituent elements of the compound can be controlled precisely. A combination of the MBE and the reaction-evaporation is much effective for some compounds. D. G. Schlom et al. "Molecular beam epitaxy of layered Dy--Ba--Cu--O compounds", Appl. Phys. Lett. 53 (17), Oct. 24, 1988, p. 1660-1662, disclose a modified MBE process in which a thin film of oxide superconductor of Dy--Ba--Cu--O is prepared with introducing excited oxygen gas which is activated by microwave.
In a high-temperature superconductor possessing a layered perovskite crystalline structure composed of constituent layers each containing different elements, the constituent layers are stratified successively on a substrate. Superconductive property of a final product is influenced critically by the content of oxygen atoms in constituent layers. Therefore, it is extremely important to control supply of oxygen gas to each constituent layer. In fact, it is difficult to obtain a desired crystal structure of high-temperature oxide superconductor by the known modified MBE process in which constituent layers are deposited continuously without controlling the oxygen supply, so that the quality of the final product obtained by the known modified MBE is rather poor.
An object of the present invention is to solve the problem and to provide an improved MBE process for preparing thin films of oxide superconductors of high quality and a system for carrying out the process automatically by a computer.